refactor: visual refresh — Inter font, neutral dark theme, modern component styles

CSS-layer-only changes for zero-regression visual modernization:

- Typography: Inter font with OpenType features (cv02-cv04, cv11), tighter letter-spacing
- Dark theme: neutral zinc tones (#09090b/#18181b) replacing blue-slate (#0f172a/#1e293b)
- Borders: rgba-based for subtlety instead of solid hex colors
- Cards: rounded-xl, refined shadows with dark mode variant
- Buttons: rounded-lg, subtle lift on hover, smoother transitions (150ms ease-out)
- Badges: rounded-md (softer than full-round), tracking-wide
- Inputs: rounded-lg, ring-offset-0 for tighter focus rings
- Scrollbars: thin 6px custom scrollbars matching theme
- Selection: accent-colored text selection
- Table headers: uppercase tracking-wider for modern data table look
- Utility: fadeIn animation, subtle-pulse for loading states

No component files changed — all styling propagates via CSS variables and Tailwind config.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/tailwind.config.js

@@ -4,6 +4,9 @@ export default {
   darkMode: 'class',
   theme: {
     extend: {
+      fontFamily: {
+        sans: ['Inter', 'system-ui', '-apple-system', 'sans-serif'],
+      },
       colors: {
         // Semantic surface tokens
         surface: {
@@ -53,6 +56,15 @@ export default {
       ringColor: {
         accent: 'var(--color-accent)',
       },
+      boxShadow: {
+        'card': '0 1px 3px 0 rgba(0, 0, 0, 0.04), 0 1px 2px -1px rgba(0, 0, 0, 0.03)',
+        'card-hover': '0 4px 12px 0 rgba(0, 0, 0, 0.08), 0 2px 4px -1px rgba(0, 0, 0, 0.04)',
+        'elevated': '0 8px 24px -4px rgba(0, 0, 0, 0.12), 0 4px 8px -2px rgba(0, 0, 0, 0.06)',
+      },
+      borderRadius: {
+        'xl': '0.875rem',
+        '2xl': '1rem',
+      },
     },
   },
   plugins: [],